This invention relates to a scroll-type positive displacement machine, and more particularly, it relates to a scroll-type compressor whose capacity can be varied.
A scroll-type compressor is a positive displacement rotary compressor comprising two interfitting elements generally referred to as scrolls. Each scroll comprises a disk-shaped end plate and a thin-walled member, generally referred to as a spiral wrap, which projects perpendicularly from one surface of the end plate and curves outwards from the center of the end plate in the shape of an involute or other type of spiral. The two scrolls are disposed with the end plates parallel and the spiral wraps interfitting with one another so as to be in line contact with one another at a plurality of locations. The surfaces of the end plates and the spiral wraps define a plurality of spiral compression chambers between the locations of line contact between the spiral wraps. If the scrolls are rotated with respect to one another in the proper direction while maintaining the line contact between the spiral wraps, the compression chambers are gradually moved towards the centers of the scrolls with an accompanying decrease in volume. Fluid is introduced into the compression chambers at the radially outer end of the scrolls and then removed at a higher pressure from a discharge port at the center of the scrolls.
Recently, scroll-type compressors have come to be used in air conditioners. When the temperature of the room which is being heated or cooled by the air conditioner reaches a predetermined temperature, the motor for the compressor is stopped. When the room temperature again deviates from the predetermined temperature, the thermostat restarts the compressor motor, and the air conditioner again performs cooling or heating. However, when the compressor is restarted to compensate for a small temperature deviation, the air conditioner need be operated at only a very low capacity to restore the room temperature to the predetermined temperature. Nevertheless, the capacity of most conventional scroll-type compressors is fixed, and they are therefore operated at full capacity even when only a small output is required. Modern air conditioners are controlled by microcomputers which are highly sensitive to temperature variations, and the air conditioner compressor is frequently turned on and off. As a result, a large load is intermittently applied to the compressor motor. This frequent application of a large load shortens the life of the motor.
Furthermore, the ratio of the suction pressure to the discharge pressure of air conditioners varies with the room temperature and the outside temperature during both cooling and heating. A compressor is designed to run with maximum efficiency at a certain optimal pressure ratio. If the pressure ratio varies in the above-described manner from the optimal pressure ratio, power losses develop during compression and the efficiency of the compressor decreases.
In order to solve such problems, a number of scroll-type compressors having variable capacity have been proposed. For example, U.S. Pat. No. 4,514,150 discloses a scroll-type compressor with a displacement adjusting mechanism. The end plate of a fixed scroll has a plurality of holes formed therein which extend between the compression chambers of the compressor and a suction chamber. The holes are opened and closed by a control mechanism in the form of valve plates and a magnetic coil which opens and closes the valve plates. When the holes are closed by the control mechanism, the compressor operates at full capacity, and when the holes are opened, a portion of the working fluid in the compression chambers is bypassed to the suction chamber, whereby the capacity of the compressor is reduced.
U.S. Pat. No. 4,383,805 discloses a scroll-type compressor having unloader means for selectively varying its capacity. The unloader means comprises passages which are formed in the end plate of one of the scrolls and which extend between the compression chambers and a space which is at suction pressure. The passages are opened and closed by spring-loaded plunger-type valves which are operated by the application of discharge pressure to one side of the valves. With the passages closed by the valves, the compressor operates at full capacity, and with the passages open, working fluid escapes from the compression chambers to the space at suction pressure and the capacity of the compressor is reduced.
However, in both of the above-described inventions, the valve mechanism which opens and closes the holes or passageways between the compression chambers and a suction chamber is a complicated and expensive mechanism and lacks reliability. Furthermore, as the valve mechanism is bulky, the compressor becomes too large to be housed in the sealed shell of a conventional scroll-type compressor which is not equipped with a capacity control mechanism. Thus, the cost of conventional scroll-type compressors with capacity control mechanisms is extremely high.